Competition Between the Great Tit and the Blue Tit Outside the Breeding Season in Field Experiments

Great and Blue Tits coexist in the same preferred habitat. To test the hypothesis that during winter these two titmice compete we excluded Great Tits from nestboxes in two areas. In both experimental areas Blue Tit breeding numbers increased significantly, whereas in a control area no increase was observed. We provide evidence that in winter, competition between Great and Blue Tits is for roosting sites, the Great Tit excluding Blue Tits. The two species may also compete for food. We conclude that Blue Tit breeding population size is limited by interspecific competition with the Great Tit in winter.